[H-3] ADENOSINE TRANSPORT IN SYNAPTONEUROSOMES OF POSTMORTEM HUMAN BRAIN

[H-3]Adenosine transport was characterized in cerebral cortical synapt oneurosomes prepared from postmortem human brain using an inhibitor-st op/centrifugation method. The adenosine transport inhibitors dipyridam ole and dilazep completely and rapidly blocked transmembrane fluxes of [H-3]adenosine. For 5-s incubations, two kinetically distinguishable processes were identified, i.e., a high-affinity adenosine transport s ystem with K(t) and V(max) values of 89 muM and 0.98 nmol/min/mg of pr otein, respectively, and a low-affinity adenosine transport system tha t did not appear to be saturable. For incubations with 1 muM [H-3]aden osine as substrate, intrasynaptoneurosomal concentrations of [H-3]aden osine were 0.26 muM at 5 s and 1 muM at 600 s. Metabolism of accumulat ed [H-3]adenosine to adenine nucleotides was 15% for 5-s, 23% for 15-s , 34% for 30-s, 43% for 60-s, and 80% for 600-s incubations. The conce ntrations (muM) of total accumulated H-3-purines ([H-3]adenosine plus metabolites) at these times were 0.3, 0.5, 1.0, 1.3 and 5.6, respectiv ely. These results indicate that in the presence of extensive metaboli sm, the intrasynaptoneurosomal accumulation of H-3-purines was higher than the initial concentration of 1 muM [H-3]adenosine in the reaction medium. For 5-, 15-, 30-, 60-, and 600-s incubations in the presence of the adenosine deaminase inhibitor EHNA and the adenosine kinase inh ibitor 5'-iodotubercidin, metabolism of the transported [H-3]adenosine was 14, 14, 16, 14, and 38%, respectively. During these times, total H-3-purine accumulation was 0.3, 0.5, 0.5, 0.7, and 1.8 muM, respectiv ely. Thus, the apparently ''concentrative'' accumulation of H-purines can be prevented by inhibition of adenosine metabolism and, taken toge ther, these results suggest that adenosine transport in at least synap toneurosomes prepared from postmortem human brain is via a nonconcentr ative and equilibrative system.